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Natural Climate Variability on Decade-to-Century Time Scales
circulation—an idea that appears to originate with Bjerknes (1964), and one that has also been developed by many ocean modelers (see, e.g., Weaver et al., 1991; Delworth et al., 1993). This meridional overturning circulation is global, and transports heat from low to high latitudes. Thus, changes in this transport would produce high-latitude changes in the atmospheric surface temperatures at locations where the thermohaline circulation "ventilates" (in the North Atlantic and in the Southern Ocean). Such changes in the poleward heat transport in the North Atlantic have recently been observed by Greatbatch and Xu (1992). Furthermore, such changes in the heat transport could have produced the type of SST anomalies seen in Figure 6. In particular, Greatbatch and Xu found increased transports (at 54.5°N) in the 1950s and reduced transports in the 1970s, which correspond to the warm and cold SSTs seen at these times in Figure 6.
The influence of the aforementioned SST anomalies on the atmospheric circulation has been studied by Peng and Mysak (1993), among others. They showed that during the warm 1950s there was a teleconnection pattern of high-low-high sea level pressure centers established across the North Atlantic, Europe, and western Siberia. During the cold 1970s, however, there was a very large low-pressure system extending from the North Atlantic to central Europe. These two circulation patterns in the atmosphere had very different effects on the precipitation and runoff over western Siberia. Peng and Mysak found that during periods of warm SST anomalies there tended to be less precipitation and hence less runoff into the Arctic, whereas during the cold years there was more precipitation and runoff into the Arctic. One important conclusion from this study is therefore that the fresh-water budget in the Arctic is indeed affected by lower-latitude interdecadal fluctuations in the ocean. However, it does not appear that the induced changes are directly related to the interdecadal Arctic climate cycle, because the runoff into the Siberian shelf does not seem to have an immediate effect on sea-ice concentration in the central Arctic.
In a recent study by Higuchi et al. (1991), it was shown that the standing-eddy poleward heat transport in the lower troposphere in the Northern Hemisphere exhibits strong decadal-scale variability. In particular, in years of very large or very weak transports, the ice margin in the Greenland Sea is substantially reduced or expanded accordingly. Thus in this case the atmosphere is a medium that can transmit interdecadal signals into the Arctic region and possibly influence the sea-ice cover.
While many of the above suggestions are rather speculative, we believe it worthwhile to look further at connections between middle- and polar-latitude interdecadal variabilities in the climate system. In particular, we should examine the Southern Ocean and Antarctic regions for interdecadal variability to see whether there are any manifestations (e.g., ice anomalies) similar those found in the Arctic. However, one should bear in mind that if these northern and southern variations are linked by the influence of the thermohaline circulation, there will be long time lags (of the order of a hundred years) between related events in the two polar regions, because the overturning time scale of the thermohaline circulation in the Atlantic basin is a few centuries (see, e.g., Mysak et al., 1993). Also, another important factor that should be kept in mind is the different geographies of the two polar regions: The Arctic Ocean is surrounded by a land mass, whereas the Southern Ocean surrounds the Antarctic continent.
In conclusion, it perhaps is worth making one final point, which concerns both polar regions as well as the lower latitudes. It can be summed up colloquially as follows: "What sea surface temperature is to interannual variability in the tropics, sea surface salinity is to interdecadal variability at high latitudes." This contrast arises because of the very different effects of SST and sea surface salinity anomalies on the atmosphere, a relation well known to ocean and climate modelers (Weaver et al., 1991). Air temperature responds fairly quickly to changes in the sea surface temperature (on time scales of the order of weeks to months), whereas precipitation is not so immediately affected by surface salinity changes. There can be feedbacks to the atmosphere through the hydrological cycle, ocean circulation, and sea-ice formation, drift, and decay, but, as described in this paper, they are on time scales of the order of decades.
The author is grateful for financial support received over the past several years from the Canadian Natural Sciences and Engineering Research Council, the Canadian Atmospheric Environment Service, Fonds FCAR (Québec), and the U.S. Office of Naval Research. It is also a pleasure to thank Ann Cossette for typing the first draft, John Walsh for helpful discussions, Douglas Martinson for his interest in and review of this paper. The comments of Bill Chapman on an earlier draft of this paper are gratefully acknowledged.